1. Field of the Invention
The present invention relates to a device which, for the purposes of safety, passively cools gas from accidental leaks which form a gaseous "accident atmosphere". More particularly, the present invention thus prevents the stratification of incondensible gases during accidents, for example, in plants for power generation or process engineering, after escape of steam or other condensible gases from accidental gas leaks.
2. Background Information
The problem solved by the present invention will be explained with reference to the example of a water-cooled nuclear reactor.
In serious accidents, steam and possibly also hydrogen can enter the containment surrounding a nuclear reactor. Therein the atmosphere is usually air (oxygen and nitrogen), or in some cases predominantly nitrogen for maintaining an inert atmosphere. Steam and hydrogen cause both the pressure and temperature of the containment atmosphere to rise. Both effects lead to increased stresses and strains on the containment wall and, if leaks are present, act as the driving force for escape of radiotoxic substances. The simultaneous presence of oxygen and hydrogen can cause deflagrations or even detonations if the ignition limits are exceeded. These lead to further loads on the containment wall. Appropriate safety features, some of which are complex, must be provided in order to prevent destruction of plant components or of the plant itself.
Because of natural convection, and especially in the presence of hydrogen, a danger also exists of stratification or layer development in the dome of the containment. As one possible consequence thereof, the efficiency of coolers can also be greatly impaired, if they are located in zones enriched with incondensible gases.
A provision known from the prior art comprises the use of active or passive cooling devices mounted at appropriate positions (e.g., in the dome of a containment). Part of the heat conveyed into the accident atmosphere is extracted by these cooling devices and removed to the outside. Devices of such design suffer from the disadvantage, however, of higher concentrations of gaseous substances that do not condense and that lead to poorer heat removal at the heat-exchanging surfaces. Condensation of the steam contained in the accident atmosphere is effected mainly with the objective of lowering the pressure, in order to reduce the danger of leaks with the concomitant release of harmful substances. DE 31 34 019 C2 describes a device for the intermixing of gas from accidental leaks. In DE 31 34 019 C2, there is provided a blower disposed in an opening through a wall. Such a blower represents an active element, which, however, can only run on externally supplied power, even during accidents.
Spray systems wherein finely dispersed water is injected into the accident atmosphere are also known. These systems work very effectively, but are based on the principle of active safety and, usually, depend on the actuation of valves, pumps, control mechanisms and external power, as well as coolant and water supply.
DE 28 47 862 A1 (a related family member of which is U.S. Pat. No. 4,416,850) discloses such a cooling device in which cooling water is sprayed into the gas atmosphere. Downstream from the spray device there are provided a heat exchanger and a moisture separator for collection of radioactive vapors from the gas atmosphere. Finally, a device for removing the collected liquid ensures that the cooling water is removed from the gas-treatment housing.
In some cases, recycling of the collected condensate for emergency cooling purposes is provided. In this connection, the greatest possible difference between the geodetic heads of condensate source and the return can be advantageous in overcoming flow resistances, nonreturn valves and back-pressures.
Apart from the cooling and intermixing of accident atmospheres, a further problem of eliminating combustible gases and mixtures (such as hydrogen) is usually countered by means of catalytic recombiners, in which hydrogen and atmospheric oxygen, for example, flow along catalytically active surfaces and react exothermically thereon. An advantage of this method is that the hydrogen content is reduced and simultaneously the pressure is lowered. A disadvantage, however, is the additional introduction of heat into the containment atmosphere. The consequence is an increased demand for effective cooling since, at higher temperatures, the tendency toward further stratification is further increased by natural convection. Furthermore, a sufficient flow for the intermixing of the accident atmosphere is necessary to achieve a high hydrogen throughput rate.
Furthermore, DE 31 43 989 C1 discloses a recombination device for the controlled oxidation of free hydrogen inside the containment.
In DE 31 43 989 C1, lines, e.g., pipes or ducts, for the intermixing of the gas atmosphere run vertically over at least half of the containment height and are equipped with a transport device in the form of a fan. The fan transports the gas mixture from the upper region to the lower region of the containment. The recombination device is disposed downstream from the fan. The principles of passive safety are also not practiced in DE 31 43 898 C1, since an external power supply is necessary for the operation of the fan.